1. Field of the Invention
The present invention relates to a constant-frequency electric power source (Variable Speed Constant Frequency) in which an A.C. generator is driven by an engine, for example, of an aircraft, and the output frequency and the output voltage thereof are not changed even when the number of rotations of the engine, i.e. the rotation rate of the engine, is changed, and more particularly to a constant-frequency electric power source capable of supplying an A.C. electric power having desirable characteristics with respect not only to the load of a constant-frequency and constant-voltage but also to the load of variable-frequency and constant-voltage.
2. Description of the Prior Art
There has been known a conventional apparatus as shown in FIG. 1. In FIG. 1, numeral 1 denotes an A.C. generator driven by an engine (not shown) mounted on an aircraft or the like; 2, a field coil of the A.C. generator; 3A, a three-phase output coils composed of three coils 3a.sub.1 to 3a.sub.3 ; and 4, a converter unit composed of a rectifying circuit 5 and an inverter 6, i.e. the converter unit provided with controlling means of PWM (Pulse Width Modulation) or the like for achieving frequency control.
According to the conventional apparatus, the current capacity of the rectifying circuit 5 in the converter unit 4 and the inverter 6 have been set in accordance with the current capacity of the A.C. generator 1. Numeral 7 denotes voltage detecting means such as a potential transformer PT, for detecting the voltages at the input terminals of the converter unit 4, and numeral 8 denotes a generator controlling unit for controlling the current to be supplied to the field coil 2 of the A.C. generator 1 thereby controlling the output voltage of the generator 1 so as to be constant. In the conventional apparatus, the converter unit 4 receives a constant voltage, e.g. 135 volts, from the A.C. generator 1 to rectify it once to a D.C. voltage, e.g. 270 volts, by using the rectifying circuit 5, and then it is converted into an A.C. voltage by using the inverter 6 to supply a three-phase A.C. electric power P.sub.A, for example, 400 Hz and 115 volts, to a load.
The variation of voltage to be generated due to the variation of an engine power as a drive source, is compensated by the generator controlling unit 8, and further the variation of frequency is compensated by the PWM (Pulse Width Modulation) control circuit provided in the inverter 6, and accordingly the load supply electric power P.sub.A is controlled to usually maintain a constant voltage and a constant frequency. The electric power source used for some devices mounted on such as aircraft, among the various devices, require constant frequency electric power with high precision as 400 Hz, three-phase, 115 V, however most of some other ones merely require a variable frequency but constant voltage source for a heater of a device such as an de-icing device to be heated by the electric power source.
However, these devices mostly require a normal voltage higher than the output voltage (115 V) of the converter unit, and therefore it has been necessary to use a booster usually having a transformer for boosting the output voltage divided at the output terminals of the converter unit up to the predetermined normal voltage. Since the weight of the booster increases by the weight of the transformer, it has been required to improve the electric power device so as to satisfy the demand to reduce the weight of equipment to be mounted on the aircraft. Furthermore, there was the problem that some distortions in wave-form are undesirably produced in the electric power to be supplied to a second load because of using the divided voltage at the input terminals of the converter unit, thereby decreasing the quality of an electric power.